1. Field of the Invention
The present invention relates to an identification system that uses a laser retro-reflecting and modulating set. The identification depends on the use envisaged for the system: it may be a measurement of the orientation of the laser beam with respect to a reference direction related to an illuminated object, or again it may pertain to the identification of the illuminated object.
2. Description of the Prior Art
The use of transmission means, working together to remotely illuminate a set that fulfils the functions of modulation and retro-reflection, is known in the prior art.
A solution of this type is described in the U.S. Pat. No. 4,134,008, which more especially emphasizes the making of a responser in a friend or foe identification system. According to this type of approach, an electro-optical modulator, such as an interference optical filter or a PLZT electro-optical switch is associated with a mirror to create the desired functions. In these methods, the re-transmitting part is combined with a receiver, itself designed to identify the already-encoded laser wave that is received.
According to another known method, described in the French patent No. 2 433 760, an object illuminated by a laser wave comprises retro-reflecting and modulating sets in which a PLZT ceramic is associated with a catadioptric device, such as a reflecting corner cube. The application envisaged is more especially the detection and location of an object and the spatial identification of directions related to the object.
An aim of the present invention is to propose another method for the retro-reflection and modulation of the laser beam, a method which, through the parameters brought into play, can be used in a variety of applications.
3. Summary of the Invention
In accordance with the present invention, there is provided an identification system using a laser beam retro-reflecting and modulating device, the said system comprising a laser transmitter/receiver which is mechanically separate and is placed remotely from a retro-reflecting and modulating set, the transmitter producing a radiation in the direction of the said set and the receiver comprising circuits for the detection and processing of the laser rays modulated and sent back by retro-reflection. The retro-reflecting and modulating set comprises an acousto-optical deflector which periodically receives an electrical signal from a control circuit. The frequency of the said electrical signal can be adjusted to obtain the diffraction of the beam in accordance with the operating conditions of Bragg's law. The deflector is mechanically coupled with a catadioptric device placed downstream to send back the diffracted laser radiation in the same direction and, after this radiation has passed through the deflector in the reverse direction, to produce the retro-reflection of the beam towards the transmitter/receiver, the retro-reflected wave having a frequency shift with a value equal to twice the frequency of the said control signal.